Metal Distributions, Efficient n-Type Doping, and Evidence for in-Gap States in TiNiM_ySn (M = Co, Ni, Cu) half-Heusler Nanocomposites

High Resolution Image Download MS PowerPoint Slide XNi 1+ y Sn nanocomposites consisting of a XNiSn half-Heusler (HH) matrix with segregated XNi 2 Sn Full Heusler (FH) inclusions promise improvements in thermoelectric efficiencies. We extend recent research by reporting on TiNiM y Sn (0 ≤ y ≤ 1) nanocomposites with M = Co (3d 9 ), Ni (3d 10 ) and Cu (3d 10 4s 1 ). Neutron powder diffraction reveals that the Ni and Cu series produce a matrix of TiNiSn with nanosegregated TiNi 2 Sn and TiNi 1+ d Cu 1– d Sn, respectively. For the Co series, the Co inserts into both phases to obtain a TiNi 1– y Co y Sn matrix with nanosegregated TiNi 2– y Co y Sn. Systematic changes in Seebeck coefficient ( S ) and electrical resistivity (ρ) are observed in all three series. For M = Ni, changes in S and ρ are attributed to in-gap states arising from the nanosegregation. The M = Co composites show a complex interplay between the hole doped TiNi 1– y Co y Sn matrix and similar in-gap states, where the p- to n-type transition temperature increases but the maximum S remains unchanged at +30 μV K –1 . The 4s 1 electron for M = Cu is delocalized in the HH matrix, leading to metal-like ρ( T ) and up to 100% improved thermoelectric power factors compared to TiNiSn ( S 2 /ρ = 2 mW m –1 K –2 at 600–700 K for y = 0.025). These results broaden the range of segregated FH phases that could be used to enhance HH thermoelectric performance.